Method for producing an anisotropic permanent magnet material



March 3,1970 TOSHIMORI SHUIN ET AL 3,498,851

METHOD FOR PRODUCING AN ANISOTROPIC PERMANENT MAGNET MATERIAL Filed Dec.17, 1964 United States Patent US. Cl. 148 101 4 Claims ABSTRACT O THEDISCLOSURE x A continuous method for obtaining a permanent magnetmaterial composed of coarse crystal grains of (100) crystal orientationin the axial direction of the bar, by zone melting a bar of magneticmaterial.

The tarting material having a composition by weight of Al 69%, Ni10-20%, Co -40%, Cu 0-8%, Ti 010%, and the balance Fe is cast into a barof magnetic material, which is successively zone melted starting withthe lower end. The molten zone is translated at a constant speed (5-30mm. per minute) and the portions at a predetermined distance (30100 mm.)below the molten zone is cooled in a way that the temperature gradientalong the solidifying zone will be -30 C. per millimeter.

The bar of magnetic material thus processed is solution treated at about1250 C., and then is aged at about 600 C. There is produced a permanentmagnet material having a magnetic characteristic of (BH) max.

7.5-85x10 gauss oersted.

This application is a continuation-in-part of our earlier applicationSer. No. 200,298 filed June -6, 1962, and now abandoned.

- The present invention relates to a method for producing an anisotropicpermanent magnet material composed mainly of Fe, Al, Ni and Co.

Alloys composed of Fe, Al, Ni and Co, or such alloys added with Cu, Ti,Nb and others are known as permanent magnet materials having excellentmagnetic property. The alloy may be given magnetic anisotropy bysubjecting it to heat treatment in the magnetic field and is,commercially used in the greatest amount. Magnetic property of a magnetalloy comprising 7.5 to 8.5% aluminum, 13.5 to 14.5% nickel, 23.0 to25.0% cobalt, 2.0 to 4.0% copper, and the balance of iron, which ismeasured in the parallel direction to the magnetic field, is shown asfollows:

Residual induction (Br) gauss 12,000 12,500 Coersive force Hc) oerstedMaximum energy'product (BH) max.

gauss oerstedQ. 4".5-5.0 10

The easy magnetized direction and the direction of dendritic growth ofthis alloy accord with each other in the 100 direction. Therefore, whenthe molten alloy is so solidified that both terminals of the magnet bodyare cooled more than other parts and subjected to, so called,directional solidification, to obtain texture of columnar crystals, theproduct has such magnetic property as 12,500 to 13,500 gauss Br, 600 to650-oersted .permanent magnet alloys used industrially.

Various improvements have been tried for the process of directionalsolidification. It is an important point of improvement of permanentmagnet alloys to make coarse "ice directionalized columnar crystalsthereby to make the deviation from the l00 direction as small aspossible.

That is, it is described in Japanese patent publication No. 4,903/ 1953that direction is given to cooling by a chill plate thereby to definethe solidification direction in the case of casting those permanentmagnet alloys, and the method has been developed with regard tomanufacture and magnetic property. Patents of Japanese patentpublication No. 2,754/ 1958, Japanese patent publication No. 6,706/1958, Japanese patent'publication No. 6,505/ 1959, and Japanese patentpublication No. 461/ 1960 are concerned with such methods. I

All of these patents have the same object in regard to point ofdirectional solidification, however,they are different mainly in theprocessof casting, i.e., position of a chill plate, form and a method ofdividing a magnet.

On the other hand, such an' improvement as to form a magnet in the formof a bar having the texture of columnar crystals and to divide it forproducing the product has been conducted apart from the above-describedimprovement. That is, according to the method of Japanese patentpublication No. 5,258/ 1959, in an improvement of a process forproducing an anisotropic permanent magnet material having texture ofcolumnar crystals, the melt of a magnetic alloy is fed in a tube, and iswithdrawn from the bottom terminal in the form of a solid bar. Thedistribution of temperature in the tube end the withdrawal speed of thesolid bar are controlled,

whereby columnar crystalline structure is given and an excellentmagnetic property may be obtained.

According to the method of Japanese patent publication No. 14,682/1961,the magnetic property is improved by casting a magnet alloy in form of ahive, and cooling the bottom of the moulding rapidly to form a magnet inform of a bar which has the texture of columnar crystals,

In spite of these improvements, columnar crystals perpendicular to thesolidification direction are still contained in an amount of about 20 to30% by volume. Then, a great amount of small crystal grains are formedeven in the columnar crystals in the solidification direction, and thesecrystals are inclined to have great the deviation from the l00direction. That is, deviation of direction of crystals and confusionof,magnetic block in the grain boundary are still present in-suchalloys, which have an injurious effect on the magnetic property. Infact, it has been reported that the magnetic property of a singlecrystal of the alloy in the l00 direction are as follows: (BH) is 7.5 to8.5 X10 gauss oersted. In case of adding Ti into the alloy, the crystalsbecome fine, so that it is quite difiicult to obtain texture of columnarcrystals and the effect of directional solidification is re duced.

It is known that as coercive force is increased effectively by additionof Ti, a single crystal of an alloy to which 5% of Ti is added shows thehighes (BH) i.e. 110x10 gauss oersted, of any kinds of permanent magnetalloy. Accordingly, though the magnetic property of a single crystal ofan alloy of the system is excellent, the property of a magnet which isobtained therefrom actually is deficient.

The present method resides in providing the re-orientation towards l00direction of crystallization of permanent magnet alloy and crystal graincoarsening.

In the first stage of the study of the subject matter of theapplication, applicants adopted a process of melting a bar of a magnetalloy with a high frequency induction coil in a zone and moving themolten zone without quenching. In this case, with a travelling rate ofless than 5 mm./min., there is obtained a columnar crystal 2-8 mm. indiameter and 50 mm. in length, which is a very coarse crystal grain,that is, developed remarkably in the axial direction, and this tendencywas greater when the travelling rate decreases, but the orientation ofthe crystal grain toward 100 direction was insuflicient. In the nextcourse of study, as a result of quenching the edge of molten zone bychill plate or oil, the crystal grain is elongated in the axialdirection, namely, 3-7 mm. in diameter and 60-100 mm. in length andshowed an orientation almost conforming with l direction. Furthermore,when the travelling rate of the-moltenzone is increased, that is, to10-20 mm./min., in the next stage, the portion of the molten zone wascooled with water, oil and water chill plate, the crystal grain was '1-4mm. indiameter and 50-80 mm. in length and the orientation to l00-direction of the crystal grain was improved remarkably. Thus, therewasdiscovered how to obtain industrial production of magnetic materialhaving superior magnetic properties. 1

The present 'invention'relates to a method for producing an anisotropicpermanent magnet material from an iron-base alloy containing aluminum,nickel and cobalt as the principal alloying elements,-which comprisesthe steps of: preparing an elongated bar of said alloy, heating aportionof said bar to a temperature above the melting point of saidalloy to melt said portion and to form a molten-zone in said bar, movingthe formed molten-zone in one direction along the axis of said bar at apredetermined travelling rate and cooling the region of said molten-zoneadjacent to the rear boundary thereof by passing said region at theabove said predetermined travelling rate through a cooling means havinga predetermined temperature gradient to solidify the molten alloy sothat it forms a bar of permanent magnet material having columnar crystalstructure oriented in the axial direction of said bar.

A magnetic alloy to be used in the present invention as the startingalloy comprises Al 6-9%, Ni 10'-20%, Co 15-40%, Cu 08%, Ti 0-10% and thebalance of iron, particularly preferably Al 7-8.5%, Ni 13,15 Co 20-38%,Ti 0-6%, Cu 2-4% and the balance of iron.

The temperature of the molten zone is selected depending upon thechemical composition of the magnet alloy employed as the magneticmaterial, and a temperature higher than the melting temperature by 50 to100 C. is preferable. Accordingly, the temperature of a magnet alloy ofAlnico 5, in which Ti is not contained, is 1450 to 1650 C. and in caseof a magnet alloy of Alnico 12 containing Ti in an amount of about it is1350" to 1550 C.

In the process of the present invention, the heating zone through whichthe magnetic alloy to be treated is passed in order to heat said alloyto the melting temperature thereof and to form the molten zone byzonemelting method and the cooling zone through :which the molten alloyin the molten zone is passed in order to solidify said molten alloy andto cool the solidified alloy at a predetermined cooling rate are soarranged as to be positioned on the opposite sides of said molten zone,respectively. The heating zone generally has an average temperaturegradient of 20 C./mm. along the path of the alloy in the temperaturerange of 200C.- 1500 C., and the cooling zone has in general an averagetemperature gradient of -30 C. along the path of the alloy in thetemperature range of 150 C.200' C.

The anisotropic permanent magnetmaterial produced industrially by thepresent invention is in the shape of bars having various sectionalshapes (for example, circular, multiple, etc. and the sizes thereof are1-"300'mm; in sectional'areas, 300-2000 mm. in length, in general thosehaving 100-500 mm. in sectional areas and 500- 1000 mm. in length aresuitable. The volume of molten alloy in the course of execution of thepresent invention is the volume of the portion in the zone formof thebar material and can be computed by the multiplication of the sectionalarea of the refractory tube (which Assuming hat the section is circular,in general the width of the molten zone corresponds to the diameter ofthe material. (However, when the sectional area is very small, or isvery large, the relation changes and when the sectional area is 1-3000mm. the width of molten zone becomes 3-50 mm. For-example, whenobtaining a circular product of diameter 20 mm., the width of moltenzone is about 20 mm., "therefore, the molten volume is about 45 grams(supposing the density is 7.3). Additionally, when the section of therefractory tube is other than a circle, theparameters correspond to theforegoing. 1

The holding time in the molten state in the course of the execution ofthe present invention can be determined bythe width of molten zone andthe travelling rate. In a circular product of a diameter of 20 mm.,"whenthe width of molten zone is about 20 mm. and-the travelling rate is 10mm.'/min., the holding time is 2 min. in all parts.

As above stated, the molten volume and the-holding time in eachalloy arevaried by the-sectional shape oft the product and the latter-is furthervaried by the travelling rate. In addition, the size of alloy to be usedas the starting material may be in any form or size which'issuitable'for the handling. t

In the case where the travelling rate of the molten zone is too high,the diameter and the length of the columnar crystals are short and themagneticproperty becomes inferior. While, in case where said rate is toolow, quite excellent magnetic property which is similar to property of asingle crystal may be obtained, but productivity is small and thereforeit is expensive. Accordingly, the travelling rate is suitably selectedaccording to the kind of the alloy and its use. Generally speaking, saidtravelling is conducted at the rate of 60 to 1200 mm./hr. In the presentinvention, the moving of the molten zone of the magnet alloy may beconducted more than twice to make the magnetic property thereof moresimilar to that of asingle crystal. By moving the molten zone severaltimes, the travelling rate may be higher without spoiling the magneticproperty, and besides its productivity may be improved. In the processof the present invention, the material charged in the tube is meltedfrom one end by the high frequency induction coil in a zone and themolten zone is gradually moved. The portion passed through the moltenzone becomes the cooling zone and the portion to be quenchedsubsequently becomes the heating zone. The cooling zone is not cooled bynatural cooling, but is cooled 'by cooling water, cooling oil, awater-cooling jacket, or an air stream, whereby the portionof thecoolingzone carrying out the forcible quenching becomes naturally theedge of the cooling zone. j

v The temperatures of respective portions in the present method are,1350-1500 C. In the molten zone, 150?- 200 C. In the edge of the coolingzone, and the distance between them is 5-150' mm., but preferably 30-mm. I

According to the method of the present invention, the magnetic propertyof 'said allow is extremely improved and the productivity is greatlyincreased by conducting continuously the process for production of saidalloy using starting magnetic alloy in different conditions, whereby amagnet having high performance maybe produced at low cost.

The starting alloys for producing magnets by thepresent method may be inany form of a bar, a fragment, powder or a melt, and therefore may beselected freely by the condition of starting alloys employed or theproducing method. v

Furthermore, a magnetmaterial in the form ofv a bar having. anisotropicmagnetic property maybe directly produced from the starting alloys, sothat the productivity is extremely greater compared withthose of theconventional sand casting method and shell, casting m th d,

. Moreover, the operations of supplying the starting alloys andproducing the product may be multiplied in asingle apparatus controlledby a smaller labor force by' 'automatic control of electric heatingsystem. Accordingly, the productivity is quite high. Then, themagneticfieldcooling and annealing of the later process may becontinuously carried out, which is very convenient'forthe pro ducingmethod. V i I As the heating method, a high frequency induction heatingmethod is generally employed, however a metallurgical heating method, SLlCh as electron beam melting method, electric arc melting method, orelectric resistance heating method may be utilized. Themolten zone iscontaminated by oxygen in air. Therefore, it is desirable to. conductthe production'jin a non-oxidizing atmosphere, such asvacuuin, argon,orhydrogen atmosphere, where oxygen is absent or reduced'." I

The essence of the present invention is to mel't'a magnet alloy bya'high frequency induction coil in a 'zone' and to move't he moltenZone' and to "quench at once' the co mpletely molten portion. f

' The molten zone may be moved eitherIby' moving. a refractory tubewhich includes the material or bymoving the high frequency inductioncoil. Since the 'distance to the cooling zone is constant, thereis'fnofditfere'nce of effect on'the quality of the'products and as tothe moving mechanism,the known fine feeding apparatus can be appliedwithout any difficulty. I

The producing method of the present invention is considered to besimilar to the method described in Japanese patent publication No.5,258/1 959. However, the former is quite different from thelatterinssubject matter -.and concrete processes. That is, the method of. theabovementioned patent publication No. 5,258/19591is a] kind of acontinuous casting method, which comprises feeding tnolten alloys into atube and solidifying it therein. Control of said alloy with regard 'todirectional solidification can be conducted only in such'degreeas;maintaini n'g the'temperature of the tube,vand; in connectionwith-it, control of withdrawal speed of the alloy in form of a bar islimited itself.

On the other hand, according to the present method, permanent magnetmaterials having desired magnetic properties may be produced by properlycontrolling the I heating system for positively melting and moving themolten Zone in the tube and-by controlling thetravelling rate of themolten alloys. a It has been found that forming of segregation which isa problem for the producing method of the present invention is notbrought up in point of magnetic property, since the travelling rate ofthe molten zone is high. Magnet material in the form of a bar producedby the present invention is not contaminated by oxygen duringmelting, sothat its surface is smooth and may be easily cut by a high speed cutteror a diamond cutter, or by subjecting it to discharge processing.

A particular characteristic of the present method resides in that theproduct has a great effect when it is used as a small-sized permanentmagnet having high performance or a high performance magnet of crosssectionally different forms, and that the product has remarkablyexcellent magnetic property, compared with products by conventionalmethods.

Referring to the attached drawing, the present invention will further byexplained hereinafter.

The sole figure of the drawing diagrammatically shows in cross-sectionan example of an apparatus used in an embodiment of the presentinvention.

Starting materials of a magnet alloy 1 is fed into a refractory tube 4by feed rolls 2, preheated by heating wires 5, a portion of saidmaterials being melted to form a molten zone 7 of belt form by heatingsaid portion with a high frequency induction heating coil 6, whileintroducing inert gas into the tube from gas-introducing openings 3. Themolten materials in said molten zone are cooled slowly bypassing themthrough heating Wires 8 and gradually solidified. A solidified magnetalloy 12 is subjected to forced cooling by passing it through a coolingzone formed by a 'water jacket 10 and a water bath 11 provided along theouter periphery of a pipe 9, and gradually taken out by leading rolls13. Thus, the molten zone is moved gradually with respect to thematerials of a magnet alloy along the axis thereof by the relativemovement of the materials 1 and the coil 6. The molten zone may also bemoved by stopping operation of rolls 2 and 13 and moving the coils 6.

The method for producing anisotropic permanent magnet materials ofthepresent invention is illustrated in the following examples.

EXAMPLE 1 .Fragments of a magnetic alloy comprising by weight 8% Al, 14%Ni, 24% C0, 3% Cu and the balance of Fe are heated at a temperatureranging from 1450 C. to 14 80 C. in an argon atmosphere by highfrequency induction coil, to melt them in form of a zone. Then, a magnetmaterial in form of a bar is withdrawn by leading rolls in such way thatthe molten zone is moved at a travelling rate of 360 -mm./hr. and theportion where the molten zone is passing is cooled.

Distance from the lower part of the molten zone to the upper part of thewater cooling zone is about 60 mm., the temperature of the side of thebar in the water cooling zone is 150 C. to 200 C. and therefore thetemperature gradient between them is 20 to 23 C./mm. The thus producedmagnet material is in the form of a bar of diameter 10 mm. is subjectedto solution treatment at 1250" C. and subsequent aging at 600 C. for 6hours, thereby to obtain an anisotropic magnet material. The magneticproperty in the axial direction of said anisotropic magnet material iscompared with those of magnet alloys comprising the same componentswhich are produced by the conventional shell casting method and the sidechill process, as follows:

Br, gauss BHc, oersted- (BH)max., g.0

Present: method. 14, 730 8. 4X10" Conventional methods. 13, 000 650 5. 9

EXAMPLE 2 A magnetic alloy in the form of a bar is produced from thesame components withthose in Example 1 and fed into a tube of diameter 3mm. made of refractory materials. Thereafter, the molten zone is movedat the rate of 360 mm./hr. by moving the high frequency induction coils.The temperature gradient from the molten zone to the cooling zone is 25to 27 C./mm. Thethus produced magnet material in the form of a bar ofdiameter 3 mm. is subjected to solution treatment as defined in Example1 to make an anisotropic magnet material. The magnetic property in theaxial direction of this magnet material is compared with that of amagnet alloy comprising the same components by the conventional shellcasting method, as follows:

A magnetic alloy is produced by melting 7% (by weight) A1, 13.5% Ni, 30%Co, 3% Cu, 5% Ti and the balance of Fe in a hydrogen atmosphere by highfrequency induction coils, followed by melting them in form of a zone.Then, the molten zone is moved at the rate of 360 mm./hr. and theportion where the molten zone is passed is cooled in a water coolingdevice made of a copper jacket and a water tank to make magnet materialin form of a bar of 15 mm. diameter.

The thus prepared magnet material is again melted in a zone in the sameway as described above, and the molten zone is moved at the rate of 360mm./hr. and cooled in the same Way as mentioned above, the produce amagnetic material. The magnetic material is subjected to the solutiontreatment defined in Example 1, thereby to give it anisotropic magneticproperty in the axial direction. Said magnetic property is compared withproperties of magnets of the same components produced by conventionalshell casting method and the side chill process.

, What we claim is:

1. A method for producing an anisotropic permanent magnet material froman ion-base alloy containing aluminum, nickel and cobalt as theprincipal alloying elements, which comprises the steps of: preparing anelongated bar of said alloy having a length of about 300-2000 mm. and across-sectional area of about 1-3000 mm. heating a portion of said bar,to a temperature above the melting point of said alloy to melt saidportion and form a molten-zone in said bar, said portion being heatedabove the melting point by an electrical heating means a temperature of1350-1650 C. to form the molten-zone with a band width of about -50 mm.,moving the thusly formed molten-zone in one direction along the axis ofsaid bar at a travelling rate of about 3-30 mm./min., applying heat tothe molten-zone after passage thereof through the electrical means at atemperature less than the melting point of the alloy to cause the alloyat least to partially solidify in said molten-zone at a slow rate, andthereafter forcibly cooling the now partially solidified zone by -80 mm.length of 3-8 mm. diameter having a crystal orientation or (1 00)substantially parallel to the axis of s aidbar.;l t J H 2. A method, asclai'rnedin claim '1 wherein said alloy consists by Weight of 6-9% ofaluminum, 10-20% of nickel, ,15-40'% of cobalt, 0-8 of copper and thebalance of iron, saidi portion of the alloy being heated at atemperature of between 1450 and 1650 with a high frequency inductionheating coil to produce saidfrr'ioltenzondfthe'c'obling means, being ata temperaturev between -200? C.', the travelling, rate being 5-30mm./rnin., and the. distance between the end of the induction coil andthe, beginningjof. the cooling means being 5 to 150mm.

\ 3. AI method as claimed in claim 1 wherein said alloy consists by"weight. or 69% of aluminum, 10-20% of nickel, '1540,% OfICObZllI,,"O-8%of copper, 0-10% of titanium and the balance of iron, said portion ofthe alloy being heated at a temperature of between 1350'and 1550 C.,With'a high frequency induction heating coil to produce saidmolten-zone, the coolingmeans being ata temperature between 150-200 C.,the travelling rate being 3-15 mm./min. and the distance between the endof 'the' induction coil and the beginning of the cooling means being5-150 mm. i

v 4. A .rriethod accordin'g a) claiinl, wherein the elongatedfbar ofsaid, alloyis pass'ed in the heating zone through a cylindricalhollowsheathof refractory material.

References Cited 7 HYLAND BIZOT, Primary Examiner r Assistant Examiner,t f jusifci. X.R. 148102, 103, 108'

7.5-8.5X10**6